Jesus Christ is “the Son of God, the Father of heaven and earth, the Creator of all things from the beginning” (Mosiah 3:8). He is Father and He is Son, depending upon which of his roles and functions we are viewing at the time.
For a detailed discussion of the ministry of Christ as the Father and the Son, see Commentary 2:225-30. in short, Amulek is declaring that Christ is the God who should come (verse 27) that he is the Son of God (verse 33), and that he is the Father of heaven and earth (verse 39).